A trapping prevention guard is necessary in motor-driven seat adjusting devices in motor vehicles, for example in window winders, sliding roofs, sliding doors, tailgates etc, for safety reasons, in order to stop and possibly reverse the motorized drive when necessary, that is to say if an object or body part is trapped. Trapping prevention of this kind is in particular also desired in motorized seat adjustment means. Characteristic variables of the motorized drive are normally evaluated in order to determine whether trapping has occurred. Such characteristic variables are, for example, the motor voltage, the motor current or the rotation speed. The motor moment is normally determined from these characteristic variables, and an excess force is determined from the motor moment in turn. The excess force is given by the difference between the total force exerted by the motor and a total adjusting force which is required, in particular, to overcome the friction and to accelerate the adjusting device. However, it is difficult to determine the adjusting force since, for example, the friction can vary during the course of the adjustment process on account of areas with severe running difficulties. In addition, aging effects or else temperature influences can have a considerable influence on the friction. Temporarily varying acceleration forces are also taken into account when determining the excess force. Thus, for example, according to EP 1 310 030 B1, a large number of individual forces are added up at a summation point in order to determine the resulting excess force and an excess force or a trapping force is determined by comparison with the force currently exerted by the motor.
EP 1 299 782 B1 discloses a trapping prevention guard in which the current profile of the force exerted by the motor over the adjustment path is compared with the profile of the force of a previous actuation process. However, if there is a relatively long period of time between the two actuation operations, the ambient conditions, for example temperature, may have significantly changed. Employing the force profile of a previous actuation process may therefore prove problematical in order to be able to use the previously measured force profile as the current profile of frictional force.
EP 0 714 052 B1 discloses a trapping prevention guard for a side window or a sliding roof, in which the adjustment process is divided into equal time windows which lie in the region of 100 msec. In this case, this time window width should be selected on the basis of the trapping instance to be detected which occurs most slowly. In order to determine the excess force, the measured values of the current time point are compared with those of a reference time point which is at a distance of one window width from the current measurement time point and at which trapping has not occurred.
Reliable identification of trapping prevention in the event of seat adjustment is more complex than in relation to window winders or sliding roofs, in the case of which the glass pane moves toward a fixed stop.